[FLINK-3869] WindowedStream.apply with FoldFunction is too restrictive

[ghost]

This pull request removes the restriction that a WindowFunction in a fold/apply should emit the same type as the accumulator type of the fold. It brings the following changes to the Java and Scala APIs:

• Deprecating apply(ReduceFunction, WindowFunction) and the restrictive apply(Initial, FoldFunction, WindowFunction) methods.
• Replacing them with two methods : reduce(ReduceFunction, WindowFunction) and a non-restrictive fold(Initial, FoldFunction, WindowFunction).
• Changing some test classes to use the new methods.

[aljoscha]

I merged this. Thanks a lot for your work! 😃

Could you please close this PR if our bot doesn't?

[ghost]

Thanks @aljoscha for looking at it and for merging. Closing the PR.